Low-speed modems are easy to design
with ICs. They can perform all of the necessary
functions, from tone generation to signal conversion.
Fur some engineers, the design of a data modem
(modulator demodulator) is still a major project.
But in low-speed applications, in which simple fre-
quency -shift keying (FSK) suffices, all of the
olucks m the modem biock diagram can be realized
by using one or more ICs (Fig. 1).
Line drivers and receivers that conform to FIA
specification KS-232 (see box) are available in
quad Its (foui drivers or receivers per package).
1 he control logic and signal-converter circuitry
usually consist of an assortment of standard digi-
tal It h. With perhaps an up amp included in the
signal converter to provide it with a frequency-
discrimination capability. And the main compo-
nents in the tone generator and the active filters
arc usually a bunch ot IC op amps.
Hysteresis needed in line receiver
Since the line receiver will usually be pm chased
as a single component, it is more important to
Know how to buy one than how to build one.
One important tact to boar in mind is that most
modems have eight to 16 lines connecting them
with the computer terminal. To save space, there-
lore, it ts desirable to use lme-recei ver chips that
contain several complete receivers per package.
A second important characteristic to look for is
high input hysteresis. Without it the line re-
ceiver will be too susceptible to input noise. This
point is so important that the Motorola MCT48i)A
line driver (Fig, 2) has been designed to have
more than four times as much hystersis as its
predecessor, the MCT489. The later model has a
typical turn-on threshold of 2 V, but it doesn't
tum-off until the input voltage falls below 0.8 V.
The earlier units had only 250 mV of hysteresis.
The entii e hysteresis loop is above ground, so
that the receiver meets the fail-safe requirements
of RS3-232-C This means the receiver output al-
ways goes to a mark condition when the input
sees an open circuit.
A third desirable characteristic is a provision
Clay Tatom, Manager, Communications Marketing, Motor-
ola Semiconductor Products, Inc.. P.O. Box 20906.
Phoenix. Ariz 85036
What is EIA spec RS 232?
The Electronic Industries Association (EIAT
has standardised a specification to which the
voltage and the impedance levels of the ONEs
and ZEROs on the computer side of the modem
must conform so that any modem can talk to
any computer with which it might be mated.
So, the RS-232 specification standardizes the
voltage and impedance levels at the computer-
modem interface (not at the modem-telephone
interface). Two different versions of this speci-
fication exist: the older RS-2-52 B spec and the
newer KS-2:'>2-C. Most modems being designed
today for industrial use conform to the RS-232-C
spec. The mu.~t important features of this speci
fication arc given below.
Driver output logic levels
with 3k to 7k load
Driver output voltage with
Driver output impedance
with power off
Output short circuit
Driver slew rate
Receiver input impedance
Receiver input voltage
Receiver output with open
Receiver output with 300
ohms to ground on
Receiver output with
+ 3 V input
Receiver output with
-3 V input
15 V > V„„ > 5 V
-5V> V UL > -15 V
j V., ■ < 25 V
Z > 300 ohms
IU < -5 A
dv/dt < 30 V/ M s
7k ohms > R,„ > 3k
^25 V compatible
RS-232-C -15 *
Ellciromc Disigis 18. September 2. 1971
1. Each of the blocks in this modem can be realized
with one or more ICs. Only the tone generator and filters
will need some passive components. When working with
the switched telephone network, a Data Access Ar-
rangement must connect the modem with the network.
2. This line receiver has over a volt of hysteresis. The
response-control input can be used to shift the input
threshold by returning it to an auxilliary supply voltage
through an external resistor; it can be used to provide
extra noise filtering by returning it to ground through
an external capacitor; or it can be used to connect two
receivers in parallel to the same input line by tying the
line to them through 8-kn resistors.
for threshold shifting. This enables the unit to be
used in the control-logic circuitry as a level
translator between MOS devices and TTL or DTL
The control logic itself is usually a section of
TTL, DTL or MOS logic circuits that control the
tone generator, FSK, and or signal-conversion
stages of the modem. The actual logic varies with
the modem manufacturer, but most use this con-
trol section to gate the FSK switches on or off
and to drive the inputs of the line drivers.
This FSK unit uses four op amps
Although highly sophisticated modulation tech-
niques are needed for high-speed data transmis-
sion, most low-speed systems being built today
use simple FSK tone generators, such as the one
shown in Fig. 3. The circuit features an integra-
tor Schmitt-trigger loop with fourth-order filter-
ing for the output signal. A pair of dual op amps
is used to minimize the parts count. Good stability
is ensured by the zener clipping at the trigger
Variable frequencies are generated by changing
the input transconductance to the integrator
through a Tee network. Shunt switches may then
be used to frequency shift from 1070 Hz to 1270
Hz, for the example shown. The integrator ampli-
tude is constant with this approach, and its har-
monics are easily filtered.
The filter employed is a modified double-sec-
tion, second-order Butterworth type, with slight-
ly more peaking than normal to equalize the sig-
nal amplitudes and still retain maximum
harmonic suppression. Ordinary Sallen-Key type
low-pass niters are used since the corner frequen-
cies are not critical and the relative Q is 1.
The active-filter section of the modem is
probably the most controversial of all the blocks.
Electronic Design 18. September 2. 1971
001 /IF ;
^— | O.OOI/iF^ — IFl
i 1 i* V 1
3. This FSK tone generator has good stability because
of the zener-diode clipping at its input. The output fre-
quency shifts from 1070 to 1270 Hz when the input
4. Here's one way to build a low-speed signal converter.
This circuit is essentially the opposite of a tone gen-
erator — its output changes logic levels when its input
changes from a mark to a space. Each of the op amps
is half of an MC1458G. The waveform shows phase con-
tinuity during tone switching.
longer has to select resistors, capacitors, and or
inductors to 1% or 0.1% to achieve his filter
characteristics. No longer does he have to worry
so much about component drift with temperature
either. This is the approach Motorola engineers
recommend for most active filters in a modem.'
The signal-conversion block is almost as con-
troversial as the active-filter block. Most modems
are built with TTL or DTL logic circuits perhaps
combined with an operational amplifier which
provides frequency-discrimination characteristics.
This stage takes the audio tones coming from the
active filter and demodulates them to give either
a ONE or a ZERO depending upon which fre-
quency tone is present. Usually the output is at
TTL logic levels. Like the active-filter section,
designs are so varied that no standard circuit ex-
ists for this block of the modem, although a
typical circuit might look like that of Fig. 4.
Designers of active filters usually have their own
preferred technique. So long as it works for them,
they are often not inclined to explore the advan-
tages of other approaches.
Motorola has worked with several techniques.
One is the Sallen-Key method shown in the tone
generator. Another is the "bi-quadratic" tech-
nique, which has been around Bell Telephone Lab-
oratories a long time but has become popular only
recently, because the low-cost IC op amp made it
economically feasible. This technique requires
several active elements instead of only one, but
this trade-off today usually results in production-
cost savings as well as other advantages.
The big advantage of the "bi-quadratic" filter
is that when the designer uses the several active
op-amp elements that the design requires, he no
Don't let the driver slew too fast
The last block of Fig. 1 that remains to be dis-
cussed is the RS-232 line driver. As with the line
receiver, quad packaging is desirable since eight
to 16 lines usually have to meet the RS-232-C
A point worth noting is that RS-232-C specifies
a maximum slew rate of 30 V. /xs. Some drivers,
such as the Motorola MC1488, will exceed this
rate if they are made to work into less than 330
pF of capacitance. For very short lines, therefore,
an external capacitor may be needed to roll the
slew rate down to the 30 V/jus maximum. ■■
1. Motorola Integrated Circuits for Modem and Ter-
minal Systems, 1970.
ELECTRONIC Dlsign 18. September 2, 1971